Sulfur recovery refers to the conversion of hydrogen sulfide (H2S) to elemental sulfur. Hydrogen sulfide is a byproduct of processing natural gas and refining high-sulfur crude oils. The conventional method of sulfur recovery is the Claus process. Approximately 90 percent to 95 percent (%) of recovered sulfur is produced by the Claus process. A conventional Claus process can recover between 95% and 98% of the hydrogen sulfide.
The conventional Claus process includes a thermal combustion stage and a catalytic reaction stage. In terms of equipment, the Claus plant (Claus unit) includes a thermal reactor and two or three catalytic reactors (Claus converters). Typical sulfur recovery efficiencies for Claus plants with two Claus converters (reactors) is between 90 and 97%, and for a Claus plant with three converters between 95 and 98%. But there is increasing demand to achieve higher sulfur removal and recovery efficiency due to tight emissions regulations. Recent environmental regulations regarding sulfur oxides (SOx) emissions place a stringent requirement on commercial sulfur recovery and accordingly, most countries require sulfur recovery efficiency in the range of 98.5% to 99.9% or higher.
The addition of a tail-gas treatment unit (TGTU) can increase sulfur recovery to or above 99.9%, but requires complex and expensive equipment. The TGTU entails either an add-on unit at the end of the Claus unit or a modification to the Claus unit itself. The add-on TGTU at the end of the Claus unit is generally used when the Claus process includes two Claus converters. Although there are several varieties of tail gas treatment technologies, they can be grouped into the following four broad categories: sub-dew point Claus process, direct oxidation of H2S to sulfur, sulfur dioxide (SO2) reduction and recovery of H2S, and H2S combustion to SO2 and recovery of SO2.
Sub-dew point Claus processes are processes based on a Claus converter performing at temperatures below the sulfur dew point (lower temperature is desirable due to equilibrium nature of the Claus catalytic reaction). Sub-dew point processes provide high equilibrium conversions in one catalyst bed, but are complicated by the need for periodic catalyst regeneration by sulfur evaporation at elevated temperatures. To accommodate for regeneration, such processes are usually performed in two or three (or even more) parallel reactors, periodically undergoing reaction and regeneration. Cold-bed-adsorption (CBA) is the most efficient sub-dew point process and can achieve 99% sulfur recovery.
Processes involving direct oxidation of H2S to sulfur are based on selective oxidation of H2S by oxygen to elemental sulfur using selective catalysts.
TGTU technology based on SO2 reduction and recovery of H2S involves the catalytic hydrogenation of leftover sulfur species to H2S, absorption of the H2S with amine solution and then recycling the H2S back to the Claus furnace.
TGTU technology based on H2S combustion to SO2 and recovery of SO2 involves the combustion of leftover H2S in the tail gas stream to SO2, absorption of SO2 with a solvent (wet scrubbing), and recycling the SO2 back to the feed to Claus plant. Although SO2 scrubbing, also known as flue gas scrubbing, has not been commercially tested as a TGTU, the technology has been extensively used as flue gas scrubbing for coal based power stations.